Method of inserting/detecting digital watermark and apparatus for using thereof

ABSTRACT

For the purpose of designing watermark to be robust against image modification such as geometric modification (rotating, cutting, enlarging/shrinking, etc.), compression, and blurring, the watermark is embedded in frequency domain after formed as 2 dimensional shape, for example radial or concentric shape. In detecting watermark, it is possible to effectively detect the watermark, by using relation to a generated watermark in case where the peak is detected.

TECHNICAL FIELD

The present invention relates to a method for embedding and detecting adigital watermark in and from digital multimedia contents and anapparatus using the same. More particularly, the present inventionrelates to a digital watermark embedding and detection method and anembedding and detection apparatus using the same for producing aspatially configured watermark, embedding and recording the configuredwatermark in an image in a frequency domain, and detecting the watermarkeffectively from the watermark-embedded image.

BACKGROUND ART

Recently, together with the wide spreading of the internet and computersand the rapid distributions of multimedia data, illegal copies (piracy)and distributions are widely prevalent so that an effective protectionapparatus for a copyright to multimedia data gets required. Watermarkingtechnology is one that embeds user information (watermark),unrecognizable by a user, in multimedia data, to thereby prevent piratedcopies and protect a copyright of a copyright owner.

The watermark means a mark developed in a step using a frame forpressing wet fibrous material to get rid of water in a process makingpaper from papyrus in ancient times. Marks embedded in paper in orderfor paper manufacturers in the middle ages to prove their own goods arethe watermarks in the middle ages, and, nowadays, an image is embeddedwhich can be recognized only with light when, in a process of makingbanknotes, printing on both sides of a sheet of paper after drying thewet sheet on which printing has been done, and the image is referred toas a watermark.

In these days, together with the increase of digital media, the conceptof a digital watermark has appeared. Just as paper in an analog conceptis substituted with the concept of digital paper, digitalizing all theanalog media in which the past watermarks were embedded has brought intothe concept of the digital watermark as a mark hidden in digital images,audio, video, and so on. That is, the watermarking refers to alltechnical methods hiding and extracting a special form of watermark inmultimedia contents in order to protect a series of multimedia contents.At the beginning, researches have been carried out for methods hidingoriginal multimedia contents themselves, but, at present, it is a trendthat strong watermarking technologies using lots of technical transformmethods are developing.

The watermarking is classified into a visible watermarking and aninvisible watermarking based on the visibility of a watermark, and theinvisible watermarking is again classified into a spatial domainwatermarking and a frequency domain watermarking based on the methodsembedding a watermark.

The visible watermarking specifies a copyright by embedding in anoriginal image author information which can be recognized with eyes. Thevisible watermarking can be used with ease but has a drawback in thatthe originals are damaged.

Accordingly, the invisible watermarking is primarily used in the imagewatermarking technology in these days. The invisible watermarking is atechnology embedding a watermark not to be visually perceived by using alimit of senses of the human visual system. While the spatial domainwatermarking embeds and extracts a watermark with ease, there is a highpossibility to lose a watermark by means of signal processing, videoprocessing (non-linear filtering, rotating, cutting, moving, enlarging,and reducing transforms and the like), and compressing.

However, the frequency domain watermarking employs transform techniquessuch as Fourier transform, discrete cosine transform, or the like forembedding and extraction, so there exists a drawback in that it has acomplicated algorithm and requires lots of arithmetic operations, but ithas an advantage in that it is robust on general attacks such asfiltering or compressions.

The invisible embedding of a watermark requires an embedding of the samein a low value on a broad area, which is carried out by the spreadspectrum technology of Ingemar J. Cox. In the spread spectrumtechnology, a pseudo-random sequence is used as a watermark, which is amethod that can be effectively used since the sequence has a uniformdistribution function and is evenly distributed over the entirebandwidth of frequencies.

For methods transforming an original image into a frequency domain, thefast Fourier transform (FFT), discrete cosine transform (DCT), andwavelet transform are generally utilized a lot, which takes a methodembedding and restoring a watermark into the original state in atransform plane. However, the method has a high possibility to lose awatermark on attacks such as image rotating, cutting, moving, enlarging,reducing, or the like.

As stated above, the watermarking methods in the spatial domain orfrequency domain have advantages and disadvantages in their own ways.For an alternative, a watermarking method using the log-polar mappingand Fourier transform has been developed to compensate for the loss of awatermark, which is the weak point of the frequency domain watermarkingmethod, in rotating, enlarging, or reducing an image. The methodconverts rotations, enlargements, and reductions into a simple movementforms through the log-polar mapping and detects a watermark by using thecharacteristics that the amplitudes of the Fourier transform areinvariable with movements. However, the method is weak at the videoprocessing such as compressions as well as has a big drawback in thatthe loss due to the log-polar mapping itself is very high and theimplementations become very complicated.

As mentioned above, the developed watermarking technologies for videohave advantages and disadvantages in general in their own ways. Further,the pseudo-random sequence watermark being widely used at present canconfirm what key value a watermark embedded in an image has, but hasdifficulties in embedding and extracting various copyright information.

Further, in case of firstly casting and then embedding a watermark in aninput image, an embedded watermark is changed if the image undergoesrotations, partial cuttings, or the like, causing a problem impairingcopyright information.

DETAILED DESCRIPTION OF THE INVENTION

It is an object of the present invention to provide a digital watermarkembedding and detection method and a apparatus using the same which arerobust against image variations such as rotation, enlargement/reduction,cutting, and filtering.

It is another object of the present invention to provide a digitalwatermark embedding and detection method and a apparatus using the samewhich spatially configure a watermark, convert an image signal into afrequency domain, and embed the spatially configured watermark tothereby be robust against image variations.

It is yet another object of the present invention to provide a watermarkdetection method and a apparatus using the same which effectively detecta spatially configured digital watermark embedded in an image signal ina frequency domain.

In order to achieve the above objects, a method for embedding a digitalwatermark in an image signal according to the present inventioncomprises steps of:

using a user key and an inherent key and generating respectivepseudo-noise codes thereof;

adding the pseudo-noise code generated based on the user key and thepseudo-noise code generated based on the inherent key;

generating a digital watermark including a step of arranging in atwo-dimensional form a watermark formed by the addition;

converting an image signal from a spatial domain to a frequency domain;and

adding a magnitude component of the image signal converted into thefrequency domain and a watermark generated by the watermark generationstep.

Further, a method for detecting a digital watermark according to thepresent invention comprises steps of:

strengthening a component of the digital watermark embedded in the imagesignal;

converting the digital watermark-strengthened image signal from aspatial domain to a frequency domain and extracting the digitalwatermark included in the image signal;

generating a digital watermark for comparison with the extracted digitalwatermark;

calculating correlation between the generated digital watermark and thedigital watermark extracted from the image signal; and

detecting the watermark embedded in the image signal based on thecorrelation.

As stated above, unlike a method simply embedding a watermark of acertain form in an existing spatial domain or frequency domain, thepresent invention can embed a watermark not linearly but spatiallyconfigured in a frequency domain so that the watermark is not changeddue to external variations such as image rotation, cutting, or the like.In particular, the watermark embedded according to the present inventionis arranged in a radial form or in a form of plural concentric circlesabout the center of a block structuring an image signal from a watermarkof a stream form.

Furthermore, the digital image watermarking apparatus and methodaccording to the present invention employ a sharpness degree, a maximumvalue, and its position in use of the fourth moment (Kurtosis) in thecorrelation of a user key value and a watermark, featuring maximizingthe accuracies of the watermark detections and authentications.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objects and other features of the present invention willbecome more apparent by describing in detail a preferred embodimentthereof with reference to the attached drawings, in which:

FIG. 1 is a block diagram for schematically showing a structure of adigital watermark embedding and detection apparatus according to anembodiment of the present invention;

FIG. 2 is a flow chart for schematically showing operations of an imageconverter of a watermark embedding apparatus in FIG. 1;

FIG. 3 is a flow chart for schematically showing operations of a firstSF transformer of a watermark embedding apparatus in FIG. 1;

FIG. 4 is a view for schematically showing a structure of a watermarkgenerator of a watermark embedding apparatus in FIG. 1;

FIG. 5 is a view for showing an example of a two-dimensional watermarkimplemented by a watermark configurer in FIG. 4;

FIG. 6A is a view for showing a process forming another example of atwo-dimensional watermark implemented by a watermark configurer in FIG.4, and FIG. 6B is a view for showing a watermark formed by FIG. 6A;

FIG. 7 is a flow chart for schematically showing operations of an FStransformer of a watermark embedding apparatus in FIG. 1;

FIG. 8 is a flow chart for showing operations of an image recorder of awatermark embedding apparatus in FIG. 1;

FIG. 9 is an exemplary view for showing filters serving watermarkdetections, wherein FIG. 9A shows a high boost filter, FIG. 9B shows aLaplacian filter, and FIG. 9C shows a DoG (Difference of Gaussian)filter having 7×7 and 9×9 masks;

FIG. 10 is a view for showing an example of a mask form employed for aneffective watermark detection;

FIG. 11 is an exemplary view for showing processed results by filters ofFIG. 9, wherein FIG. 11A is a view for showing an example of awatermarked image before filtering, and FIGS. 11B to 11D respectivelyshow the processed results by a high boost filter, Laplacian filter, andDoG filter;

FIG. 12 is a flow chart for showing operations of a second FStransformer of a watermark detection apparatus in FIG. 1;

FIG. 13 is a view for showing an example of a peak detection employedfor a watermark detection; and

FIG. 14 is a flow chart for showing operations of a watermark detectorin FIG. 1.

EMBODIMENT

Hereinafter, the watermark embedding and detection method and thewatermark embedding and detection apparatus using the same according tothe present invention are described in detail with reference to theaccompanying drawings.

FIG. 1 is a block diagram for schematically showing a structure of adigital watermark embedding and detection apparatus according to anembodiment of the present invention.

The digital watermark embedding and detection apparatus in FIG. 1comprises a watermark embedding apparatus 100 for embedding a watermarkinto an inputted image and a watermark detection apparatus 200 fordetecting a watermark from a watermark-embedded image. The watermarkembedding apparatus 100 includes an image converter 110 converting aninputted image 10 into a certain form based on characteristics thereof,a first Spatial-to-Frequency (FS) transformer 120 transforming an outputsignal of the image converter 110 from a spatial domain to a frequencydomain in consideration of an image form, a watermark generator 130generating a watermark spatially arranged, an adder 140 adding thewatermark generated from the watermark generator 130 and an image signaloutputted from the first SF transformer 120, a Frequency-to-Spatial (FS)transformer 150 transforming the watermark-added signal from thefrequency domain to the spatial domain again, and an image recorder 160recording the watermark-embedded image signal.

Further, the watermark detection apparatus 200 includes an imageconverter 210 receiving a reproduced image signal and converting it intoa certain form of format, a pre-processor 220 strengthening thecharacteristics of a watermark included in an output signal of the imageconverter 210, a second SF transformer 230 transforming the watermarkcharacteristics-strengthened image signal from the spatial domain to thefrequency domain and extracting a watermark region from thecorresponding image signal, a watermark generator 240 generating awatermark spatially arranged, a correlation calculator 250 calculating acorrelation between a watermark extracted from the second SF transformer230 and a watermark outputted from the watermark generator 240, and awatermark detector 260 detecting a watermark included in an image signalbased on an output value from the correlation calculator 250.

The operations in the watermark embedding and detection apparatus havingthe above structure are described with respective constituents thereof.First, the operations of the watermark embedding apparatus 100 aredescribed with reference to FIG. 2 to FIG. 8.

An image 10 is inputted to the image converter 110 to embed a watermarkin a digital image signal. Describing the operation flow of the imageconverter 110 with reference to FIG. 2, the image converter 110 checkswhether the inputted image 10 is a 24-bit color image (Step S100). Atthis time, it is determined by checking the header information of theinputted image signal whether the inputted signal is in 24-bit. If theinputted image 10 is in the 24-bit color, the RGB components of theinputted image are converted into a YIQ format by employing Formula 1 asbelow (Step S110), wherein Y stands for Luminance, I for In-phase, and Qfor Quadrature.

[Formula 1]

$\begin{bmatrix}Y \\I \\Q\end{bmatrix} = {\begin{bmatrix}0.2989 & 0.587 & 0.114 \\0.5959 & {- 0.2744} & {- 0.3216} \\0.2115 & {- 0.5229} & 0.3114\end{bmatrix}\begin{bmatrix}R \\G \\B\end{bmatrix}}$

The I and Q components in the converted format are separately stored,and only the Y component is extracted (Step S120). The extracted Ycomponent passes over to the first SF transformer 120.

In the step S100, if the inputted image is not a 24-bit color image, theprocess directly proceeds with the first SF transformer 120. That is, ifthe inputted image is not the 24-bit color image, the image 10 actuallyinputted corresponds to the same signal as the Y component of the 24-bitcolor case, so the inputted image 10 is directly transferred to thefirst SF transformer 120 without any conversion process into the YIQformat. Accordingly, if the inputted signal is not the 24-bit, noseparate image converter 110 may be provided. Further, in the abovecase, an input image is processed into the YIQ format based on the NTSCformat, and, in other formats, a watermark can be embedded by directlyseparating an RGB signal into respective R, G, and B channels foroutputs without converting a format of the inputted image.

A result processed in the image converter 110 is inputted to the firstSF transformer 120. The first SF transformer 120 carries out a transformin order shown in FIG. 3.

The output signal of the image converter 110 is applied withtwo-dimensional Fast Fourier Transform (Formula 2) (Step S200). A resultof the two-dimensional Fast Fourier Transform is expressed as a form ofa complex number which is divided into a real number component (R) andan imaginary number component (I). The general Fourier Transform can beemployed in lieu of the above Fast Fourier Transform.

[Formula 2]

${ 1 )\mspace{14mu}{F( {n_{1},n_{2}} )}} = {\sum\limits_{k_{2} = 0}^{N_{2} - 1}\;{\sum\limits_{k_{1} = 0}^{N_{1} - 1}\;{{\exp( \frac{2\pi\;{\mathbb{i}}\; k_{2}n_{2}}{N_{2}} )}{\exp( \frac{2{\pi\mathbb{i}}\; k_{1}n_{1}}{N_{1}} )}{f( {k_{1},k_{2}} )}}}}$M=√{square root over ((R ² +I ²))}  2)

${ 3 )\mspace{20mu}\Theta} = {{arc}\;{\tan( \frac{I}{R} )}}$

In here, f denotes an image signal, F a frequency coefficient obtainedafter Fourier Transform, M a magnitude of values obtained after afrequency transform, and θ a phase, respectively.

Since a complex number form appears by the Fourier Transform, thefrequency coefficient is divided into real number and imaginary numbercomponents. A magnitude and a phase are calculated and separated,respectively, by using Formula 2 from these components (Step S210). Inthe step S210, the image signal is separated into a magnitude and aphase respectively, and the magnitude component is FFT-shifted to beconverted into a form for embedding a watermark (Step S220). The FFTshift shifts magnitude components to a center portion in order for ato-be-later-embedded watermark to be embedded about the center. Further,phase components are transferred to the FS transformer 150 in order totransform an image signal into the spatial domain again after embeddinga watermark.

Using magnitude components for embedding a watermark is because, whenFourier-Transformed, magnitude components have the characteristics shownin Formula 3.

[Formula 3]

$\begin{matrix} {f( {{x + a},{y + b}} )}rightarrow{{M( {u,v} )}{\mathbb{e}}^{{- j}\;{({{au} + {bv}})}}}  \\ {f( {{\rho\; x},{\rho\; y}} )}rightarrow{\frac{1}{\rho}{M( {\frac{u}{\rho,}\frac{v}{\rho}} )}}  \\ {f( {{{x\;\cos\;\theta} - {y\;\sin\;\theta}},{{x\;\sin\;\theta} + {y\;\cos\;\theta}}} )}rightarrow{M( {{{u\;\cos\;\theta} - {v\;\sin\;\theta}},{{u\;\sin\;\theta} + {v\;\cos\;\theta}}} )} \end{matrix}$

Wherein, f denotes an image signal, M a magnitude of values obtainedafter frequency conversions, ρ a constant multiplied upon resizing (aresizing multiple), and θ a phase, respectively.

As seen in a result of Formula 3, the characteristics are used that amagnitude component is the same even though an image is shifted, and anabsolute position does not vary even though a image scale changes. Byconfiguring a watermark into a circular form in addition to thecharacteristics, the watermark is prevented from damages even invariations of an image by rotations.

The image signal processed by the first SF transformer 120 is added to awatermark generated from the watermark generator 130 and then embeddedto the inputted image.

The watermark generator 130 generates a watermark by a structure shownin FIG. 4. First, a user key is inputted to a pseudo-noise codegenerator 122, which generates a pseudo-noise code by using the user keyas a seed value. In the meantime, an inherent key generated tofacilitate a watermark detection, separately from the user key, isinputted into a pseudo-noise code generator 124, and a pseudo-noise codeis generated in the same manner as in the user key.

The two pseudo-noise codes so generated are added in an adder 126. Anadded pseudo-noise code is inputted to a watermark configurer 128. Thewatermark configurer 128 newly configures a watermark of aone-dimensional stream format into a two-dimensional format. Viewing aformat shown in FIG. 5 as an example, a watermark is configured to bearranged in a two-dimensional radial format while rotating 360 degreesabout a first value of a watermark of a certain length. In the case ofconfiguring a watermark in the two-dimensional radial format as above, awatermark-embedded image is not affected by external attacks since awatermark format does not changes even though the watermark-embeddedimage is varied by external attacks such as rotations and so on.

Further, another example configuring a watermark in the two-dimensionalformat is described with reference to FIG. 6. FIG. 6A is a view forshowing a process for configuring one watermark sequence into pluralconcentric circles, and FIG. 6B is a view for showing a watermark inplural concentric circles consequently generated.

As in (a) of FIG. 6A, after configuring a watermark Wseq formed in asequence of 1 and −1, the watermark Wseq is resized in various lengthsas in (b) to configure a watermark as in (c). FIG. 6B shows a watermarkimplemented by the watermark configurer through the process of FIG. 6A.

As stated above, when a watermark is configured in a circular shape, awatermark is arranged in a concentric format about the center of ablock. When arranged in the concentric format, there exists a radialdifference between an inner watermark and an outer watermark, so adifference occurs between bit lengths configuring watermarks.Accordingly, watermarks are sampled at a certain rate in accordance withradial magnitudes to be enlarged and reduced for arrangements. When awatermark is arranged in a circular shape, an upper side is configuredin a watermark stream and then the stream is copied for a lower side, tobe arranged in the same rotation direction for use. An initially formedshape is a semi-circle rather than a concentric circle, which is becausean initial component of the Fourier Transform is an origin symmetry, soa watermark is embedded by a semi-circle and then the semi-circle iscopied in the origin symmetry, to thereby bring out an effect embeddingconcentric circles as in FIG. 6 b.

The watermark so formed is inputted to the adder 140 to be added to animage signal outputted from the first SF transformer 120.

The adder 40 first divides the image into blocks of a predeterminedsize, that is, a watermark size in order to add a watermark configuredfrom the watermark generator 130 and the image signal outputted from thefirst SF transformer 120. The divided image signal and the watermarksignal outputted from the watermark generator 130 are added. At thistime, the intensity of a watermark embedded according to thecharacteristics of the image signal is determined beforehand for theaddition.

The determination of the watermark intensity can be accomplished invarious forms. For example, it is determined by the number of colorsused for each channel in a block, histogram shape, energy ratio of highand low frequencies, and so on.

For example, when dividing an image block by block, the number of colorsused for each block and a color value are obtained. In case that manycolor numbers are used and the color value is high, a real imagecorresponds to one that has severe color changes or colors of abrilliant form. Accordingly, a visual effect is not experienced so mucheven though the intensity of a watermark to be embedded in acorresponding block is high. However, in case that color changes aresmall, even a watermark embedded with a low intensity can give a feelingthat much noise is included in an original image. Therefore, the numberof colors used in a block and a color value are considered to determinethe intensity of a watermark to be embedded to be strong when the valueis high and to be weak when the value is low.

Further, if the DCT transform is applied to an image to be expressed ina block, it is characterized in that a part corresponding to a lowfrequency region is clustered in the upper left of the block, a partcorresponding to an intermediate frequency region in the center portion,and a part corresponding to a high frequency region in the lower right.That is, the DCT result enables the characteristics of an inputted imageto be grasped depending on a ratio of a low frequency energy and a highfrequency energy.

Moreover, if an inputted image is analyzed channel by channel, each ofR, G, and B channels has 8 bits (2⁸=256) and values 0˜255 are allocatedto each color in case of a 24-bit color, and it is available that ahistogram is prepared based on the values with respect to image regionsand the changes or occupied colors in an image are grasped based on theshape and changes of the histogram. That is, if the number of usedcolors is small, the distribution of the histogram becomes narrow, andto the contrary, if the number of used colors is large, the distributionof the histogram becomes wide. The large number of used colors meansthat an image has severe changes, and, to the contrary, the small numberof used colors means that an image is dull without particular changes.Accordingly, with this, it can be determined whether an image energy isconcentrated in a high frequency region or in a low frequency region.

In a method for adding a watermark, it is also possible to independentlyembed a watermark directly into each channel, that is, in case of a Grayimage, into the Gray channel and, in case of an RGB image, intorespective R, G, and B channels, without passing through the imageconverter 110. If a watermark-embedded signal is outputted, the signalis inputted to the FS transformer 150 and then converted from afrequency domain to a spatial domain.

The operations carried out in the FS transformer 150 is described withreference to FIG. 7.

The FS transformer 150 basically carries out in the reverse order theprocess done by the first SF transformer 120. That is, an FFT shift iscarried out with respect to a watermark-embedded signal (Step S300).This plays a role of shifting the signal into an original format byapplying again the FFT shift done for the signal prior to embedding awatermark. After shifting, the signal component (magnitude component)and a phase component separated and extracted before from the first SFtransformer 120 are added and then an inverse 2D FFT is applied totransform a frequency domain signal into a spatial domain signal (StepS310).

After the transform, an image signal is resized to prevent an overflowwhich can occur by an addition with a watermark carried out in the abovestep (Step S320). For example, an R channel signal of 8 bits has valuesranging from 0 to 255, which can have values less than 0 or larger than255 by the addition with a watermark in the adder 140. A watermark sizebasically has a value of −1 or 1, or, in case of resizing, has values ofinteger multiples of the above value, and, even though the size is notbig, may have a value out of a range from 0 to 255 by the addition withan image signal. At this time, abrupt color changes are developed.Accordingly, in case that an addition result becomes less than 0 orlarger than 255, an overflow occurs and corresponding values areadjusted to 0 or 255, respectively, that are boundary values the signalcan have.

As stated above, a watermark-embedded signal is transformed into aspatial domain by the FS transformer 150 and then recorded on a storagemedium and the like by the image recorder 160, such recording operationsof which are described with reference to FIG. 8.

The image recordation part 160 determines whether the watermark-embeddedsignal is a 24-bit image or not (Step S400). If the watermark-embeddedsignal is a 24-bit image, the previous IQ components left after havingextracted the Y component from the YIQ components are added to the Ycomponent (Step S410). Following the addition, a signal of the YIQformat is again converted into the RGB signal by using Formula 4 asfollows (Step S420).

[Formula 4]

$\begin{bmatrix}R \\G \\B\end{bmatrix} = {\begin{bmatrix}1.0 & 0.956 & 0.621 \\1.0 & {- 0.272} & {- 0.647} \\1.0 & {- 1.106} & 1.703\end{bmatrix}\begin{bmatrix}Y \\I \\Q\end{bmatrix}}$

A signal converted as above is stored in a storage medium in awatermarked image (Step S430).

However, if the watermarked signal is not a 24-bit image in the stepS400, the step 430 directly proceeds for storage since the watermarkedsignal is a image signal inputted from the preceding image conversionpart 110 without a separate conversion step so that the above conversionis unnecessary. Further, even in case that a watermark is embedded inevery channel process by the processing of the RGB format instead ofconverting an inputted image into the YIQ format, the step S430 directlyproceeds for storage.

The watermark embedding apparatus 100 as stated above, arranges awatermark in the two-dimensional space for embeddings and embeds awatermark in the frequency domain, bringing out an effect that awatermark does not change even when taking variations as to an imagesuch as rotations, cuttings, or the like with respect to awatermark-embedded image.

Further, the watermark detection apparatus 200 for detecting a watermarkfrom the watermark-embedded image signal as above is described withreference to FIG. 1, and FIGS. 9 to 14.

A watermarked image can flow into a pirate or an illegal user viavarious ways, be pirated, and be modified. However, in case that awatermark is spatially arranged and embedded in the frequency domain bythe watermark embedding apparatus 100 according to the presentinvention, the watermark embedded in an image has characteristics robustenough to maintain its shape even when the image undergoes variationsdue to image rotations, cuttings, or the like. Descriptions are made ona apparatus and method for detecting a watermark embedded by such amanner.

If an image in which a watermark is embedded and recorded is inputted tothe watermark detection apparatus 200, the image is first converted intoa signal of a certain form through the image conversion part 210. Thestructure and operations of the image conversion part 210 in thewatermark detection apparatus 200 is the same as those of the imageconversion part 110 in the watermark embedding apparatus 100. That is,if an inputted image is a 24-bit image, the inputted image is convertedinto the YIQ format from the RGB format, only the Y component isextracted and outputted to detect a watermark. If not a 24-bit image,the inputted image is outputted without the conversion. Further, if theinputted image is in 24 bits, the RGB signal form can be outputted as itis.

An image signal outputted from the image conversion part 210 is inputtedinto the pre-processing part 220. The pre-processor 220 is foremphasizing the characteristics of a watermark included in the imagesignal, and carries out a high-pass filtering, sharpen filtering, orhigh-boost filtering process. Such filters employed in the pre-processor220 are illustrated for examples in FIG. 9 and FIG. 10.

FIG. 9 is a view for showing examples of various spatial filtersperforming a role of boosting high-frequency components of an imagesignal, FIG. 9A, FIG. 9B, and FIG. 9C show mask forms for a high boostfilter, a Laplacian filter, and Difference-of-Gaussian (DoG) filter,respectively.

The high boost filter in FIG. 9A serves detecting a watermark, and playsa role of boosting a watermark signal. That is, it plays a role ofreducing an image component energy and increasing a watermark signalenergy. Further, the DoG filter of FIG. 9C is based on Formula 5 asfollows.

[Formula 5]

${{DoG}( {x,y} )} = {\frac{{\mathbb{e}}^{- \frac{({x^{2} + y^{2}})}{2\sigma_{1}^{2}}}}{2\;{\pi\sigma}_{1}^{2}} - \frac{{\mathbb{e}}^{- \frac{({x^{2} + y^{2}})}{2\sigma_{2}^{2}}}}{2\;{\pi\sigma}_{2}^{2}}}$

In addition to the filters in FIG. 9, a filter as shown in FIG. 10 maybe used for reducing an image component energy and intensifying awatermark component energy.

The pre-processor 220 as stated above is for intensifying a watermarkcomponent from an image signal, for which any one of the filters shownin FIG. 9 and FIG. 10 may be used for processing.

FIG. 11 is an exemplary view for showing results processed by filters ofFIG. 9. FIG. 11A is a view for showing an example of a watermarked imagebefore filtering, and FIGS. 11B to 11D respectively show the processedresults by a high boost filter, Laplacian filter, and DoG filter.

A signal passing through the pre-processor 220 is inputted to the secondSF transformer 230. The operation flows in the second SF transformer 230is described with reference to FIG. 12. The second SF transformer 230 isfor extracting an embedded watermark, which basically has the sametransform process as one of the first SF transformer 120 in thewatermark embedding apparatus 100 (Step S200). That is, the 2DFFTtransform separates a magnitude component and a phase component, themagnitude component is extracted, and then the magnitude component isFFT-shifted.

After the FFT shift, a watermark-embedded regions are extracted in onedimension (Step S234). Since a watermark-embedded position does notchange even when image transforms such as rotations,enlargements/reductions, cuttings, and so on are applied, the aboveprocessing can be carried out. The watermark-embedded regions may varyin sizes thereof. For example, in case of arranging watermarks in aconcentric shape, one can be identical to a real watermark size, butinner and outer watermarks arranged about the watermark are extended inlengths thereof and formed through manipulations such as sampling andthe like. Accordingly, the watermarks so changed in sizes are resized tooriginal sizes (Step S238).

In the meantime, the watermark generator 240 of the watermark detectionapparatus 200 is the same in a basic structure as the watermarkgenerator 130 of the watermark embedding apparatus 100, but has not thewatermark configurer. That is, the watermark generator 240 generateswatermarks cast in one dimension as to respective pseudo-noise codesgenerated by a user key and an inherent key.

The correlation calculator 250, as stated above, calculates acorrelation Corr between a watermark component of an image signalprocessed by the second SF transformer 230 and a watermark signalgenerated from the watermark generator 240 by using Formula 6 as below.Corr=IFFT(FFT(W _(EXT))×conj(FFT(W _(m))))  [Formula 6]

Here, W_(EXT) denotes a watermarked embedded in an image signalextracted by the second SF transformer 230, and W_(m) respectivewatermarks generated by using a user key and an inherent key by thewatermark generator 240. IFFT denotes a one-dimensional inverse fastFourier transform, FFT a one-dimensional fast Fourier Transform, andconj a complex conjugate.

The correlation calculations using the above Formula 6 are carried outby multiplying data obtained through the two-dimensional fast Fouriertransform with respect to a watermarked image W_(EXT) with data obtainedthrough the two-dimensional fast Fourier transform with respect to awatermark W_(m) generated by a user key or an inherent key from thewatermark generation part 230, and then the inverse fast Fouriertransform is applied to the multiplication to be converted into aspatial domain. As above, the transform into a frequency domain and thecalculations based on the multiplication reduce the number ofcalculations compared to taking convolution with an image watermarked inthe spatial domain and a watermark, enabling faster data processing.

FIG. 13 is a view for showing a floated correlation calculated based onFormula 6 as to a presumptive case that a watermark is embedded. Acorrelation obtained by Formula 6 is not a certain value, but pluralvalues in a one-dimensional sequence form, so such plural values arecompared to enable a maximum peak value and its position to be obtainedthrough a process as follows.

The watermark detector 260 checks, like a watermark is generated throughan inherent key and a user key in the watermark generator 230 if peaksoccur as shown in FIG. 14, whether these two key values exist and thepeaks occur at the same position (Step S500). A sharpness degree iscalculated based on Formula 7 if the two peak positions are the same(Step S510). The calculation of the sharpness degree, through a fourthmoment (Kurtosis) K, checks whether the value of K is more than acertain threshold value (Step S520), and it is determined that awatermark is detected when the two conditions are all satisfied.

[Formula 7]

${K( {x_{1},\ldots\mspace{14mu},x_{N}} )} = {\{ {\frac{1}{N}{\sum\limits_{j = 1}^{N}\;\lbrack \frac{X_{j} - \overset{\_}{X}}{\sigma} \rbrack^{4}}} \} - 3}$

Here,

x₁, …  , x_(N)denotes a result value of a correlation between two watermarks W_(m) andW_(EXT), X an average of

x₁, …  , x_(N),and σ a standard deviation.

The determination as to whether a value of K is more than a certainthreshold value in the above procedure is to determine whether awatermark is embedded through a comparison between a peak value and aset threshold value since a peak appears high at an calculated value incase that the watermark is embedded. In here, the threshold value isshown as a value allocated in a certain manner by experiments. However,when the condition is not satisfied in the Step S520, it is determinedthat a watermark is not detected.

INDUSTRIAL APPLICABILITY

As described above, the present invention relates to a method andapparatus for embedding and detecting a watermark, which uses awatermark formed in a radial or circular format in the two-dimensionalform upon embeddings and changes its configuration for the embeddinginto an image signal in the frequency domain, enhancing the robustnessof a watermark against signal variations. Further, upon the detection ofthe watermark, a watermark embedded in the radial or circular format inthe two-dimensional form as above can be effectively detected, enhancingthe accuracy and promptness for watermark detections.

In particular, the present invention greatly reduces the complexity ofan entire system compared to a method using the existing log-polarmapping and removes data losses which can occur in the step of thelog-polar mapping and the inverse log-polar mapping, to therebyfacilitate the watermark detections as well as remarkably reduce imagelosses when embedding a watermark.

Although the preferred embodiment of the present invention has beendescribed in particular, it will be understood by those skilled in theart that the present invention should not be limited to the describedpreferred embodiment, but various changes and modifications can be madewithin the spirit and scope of the present invention as defined by theappended claims.

1. A digital watermark embedding method for embedding a digitalwatermark in an image signal, comprising steps of: using a user key andan inherent key to generate respective pseudo-noise codes; adding thepseudo-noise code generated based on the user key and the pseudo-noisecode generated based on the inherent key; generating a digital watermarkat least in part by arranging in two-dimensional form a watermark formedby the added pseudo-noise codes, and rotating the watermark from astream format 360° about a first value of a watermark of a predeterminedlength to be in a radial format; converting the image signal from aspatial domain to a frequency domain; and adding a magnitude componentof the image signal converted into the frequency domain and thegenerated watermark.
 2. The method as claimed in claim 1, wherein thewatermark arrangement step arranges the watermark of a stream formatinto plural concentric circles about a center portion of a blockconstructing the image signal.
 3. The method as claimed in claim 2,wherein the watermark arranged in the concentric circles is arrangedwith enlargements/reductions by sampling a corresponding stream withreference to a predetermined radius.
 4. The method as claimed in claim1, wherein the frequency domain conversion step includes steps of:applying a two-dimensional Fourier transform to the image signal for theconversion from the spatial domain to the frequency domain; separating asignal converted to the frequency domain into a magnitude component anda phase component; and applying a Fourier transform shift to theseparated magnitude component.
 5. The method as claimed in claim 4,further comprising a step of converting from the frequency domain to thespatial domain the signal the watermark and the image signal are added,and the conversion step including steps of: applying the Fouriertransform shift to the watermark and the image signal; applying aninverse two-dimensional Fourier transform to the shifted signal for aconversion from the frequency domain to the spatial domain; andadjusting a value of the image signal to prevent an overflow of theconverted image signal.
 6. The method as claimed in claim 5, wherein theimage signal is a 24-bit color signal, and, before adding the digitalwatermark and the image signal, a step of converting the image signalinputted in an RGB format into an YIQ format (Y: Luminance, I: In-Phase,and Q: Quadrature) and a step of extracting only the Y component fromthe components of the converted YIQ format are further comprised, andthe Y component is added to the digital watermark.
 7. The method asclaimed in claim 6, further comprising a step of storing an image signalthe image signal and the digital watermark are added, and the storagestep including steps of: adding the IQ components and the added imagesignal; converting the added signal into the RGB format; and storing theconverted signal in a storage medium as a record signal.
 8. The methodas claimed in claim 5, wherein the image signal is the 24-bit colorsignal, and the image signal inputted in the RGB format is added to thedigital watermark by each of R, G, and B channels.
 9. The method asclaimed in claim 5, wherein the image signal adjustment step sets avalue of the image signal to a boundary value of a range in case thatthe image signal has a value beyond the corresponding range.
 10. Adigital watermark detection method for detecting a digital watermarkembedded in an image signal, comprising steps of: strengthening acomponent of the digital watermark embedded in the image signal;converting the digital watermark-strengthened image signal from aspatial domain to a frequency domain and extracting the digitalwatermark included in the image signal; generating a digital watermarkfor comparison with the extracted digital watermark by generatingrespective pseudo-noise codes and forming a digital watermark using auser key and an inherent key, and rotating the watermark from a streamformat 360° about a first value of a watermark of a predetermined lengthto be in a radial format; calculating correlation between the generateddigital watermark and the digital watermark extracted from the imagesignal; and detecting the watermark embedded in the image signal basedon the correlation.
 11. The method as claimed in claim 10, wherein theimage signal is a 24-bit color signal, before carrying out the step ofstrengthening the component of the digital watermark embedded in theimage signal, further comprised are steps of: converting the imagesignal inputted in an RGB format into a YIQ format; and extracting onlythe Y component from the components of the converted YIQ format, and thestep of strengthening the component of the digital watermark is tostrengthen a component of the digital watermark embedded in the Ycomponent.
 12. The method as claimed in claim 11, wherein the step ofstrengthening the component of the digital watermark includes a step offiltering a high frequency component of the image signal.
 13. The methodas claimed in claim 12, wherein the high frequency filtering step iscarried out through a high boost filter, a Laplacian filter, or aDifference of Gaussian (DoG) filter.
 14. The method as claimed in claim11, wherein the step of strengthening the component of the digitalwatermark includes a step of applying a masking in order to reduce animage component energy in the image signal and increase a watermarkcomponent energy.
 15. The method as claimed in claim 11, wherein thedigital watermark extraction step comprises steps of: converting thedigital watermark-strengthened image signal from the spatial domain tothe frequency domain; extracting from the converted image signal in onedimension a region in which the digital watermark is embedded; andadjusting a length of the extracted one-dimensional digital watermark toa predetermined length.
 16. The method as claimed in claim 15, whereincorrelation calculations are carried out with a Formula as below:Corr=IFFT(FFT(W _(EXT))×conj(FFT(W _(m)))) here, W_(EXT) denotes adigital watermark extracted from an image signal, and W_(m) respectivewatermarks generated by using a user key and an inherent key, IFFT aninverse fast Fourier transform, FFT a fast Fourier Transform, and conj acomplex conjugate.
 17. The method as claimed in claim 16, wherein thewatermark detection step includes steps of: calculating a peak positionfrom the calculated correlation; calculating a peak sharpness; anddetermining whether the digital watermark is included in the imagesignal based on the peak position and the sharpness.
 18. The method asclaimed in claim 10, wherein the image signal is the 24-bit colorsignal, a step of separating the image signal inputted in the RGB formatinto each of R, G, and B channels is further comprised, and the step ofstrengthening the component of the digital watermark is to strengthenthe components of the digital watermark embedded in signals separatedchannel by channel.
 19. The method as claimed in claim 18, wherein thedigital watermark extraction step comprises steps of: converting thedigital watermark-strengthened image signal from the spatial domain tothe frequency domain; extracting from the converted image signal in onedimension a region in which the digital watermark is embedded; andadjusting a length of the extracted one-dimensional digital watermark toa predetermined length.
 20. A digital watermark embedding apparatus forembedding a watermark in an image signal, comprising: a first conversionmeans for converting the image signal from a spatial domain to afrequency domain; a watermark generation means for generating awatermark; a first addition means for adding the watermark to thefrequency-converted image signal; a second conversion means forconverting the watermark-added image signal generated by the firstaddition means from the frequency domain to the spatial domain; and animage record means for recording the watermark-embedded image signalconverted into the spatial domain, the watermark generation meanscomprising: a) a first pseudo-noise code generation means for generatinga pseudo-noise code using a user key; b) a second pseudo-noise codegeneration means for generating a pseudo-noise code using an inherentkey; c) a second addition means for adding pseudo-noise codes generatedfrom the first pseudo-noise generation means and the second pseudo-noisegeneration means; and d) a configuration means for configuring in atwo-dimensional arrangement the watermark formed by the addition throughthe second addition means, the configuration means rotating thewatermark from a stream format 360° about a first value of a watermarkof a predetermined length to be in a radial format.
 21. The apparatus asclaimed in claim 20, wherein the watermark configuration means arrangesthe watermark of a stream format into plural concentric circles about acenter portion of a block constructing the image signal.
 22. Theapparatus as claimed in claim 21, wherein the watermark configurationmeans enlarges and reduces the watermark arranged in the form of theconcentric circles by sampling a corresponding stream with reference toa predetermined radius to be arranged in a form of plural concentriccircles.
 23. The apparatus as claimed in claim 20, wherein the firstaddition means divides the image signal into a watermark size, and addsthe divided image signal and the watermark channel by channel.
 24. Theapparatus as claimed in claim 20, further comprising an image conversionmeans for converting a format of the image signal before adding theimage signal and the watermark, the image conversion means including: afirst determination means for reading header information of the imagesignal and determining whether the image signal is a 24-bit color image;a first conversion means for converting the image signal of RGB formatinto a YIQ format if the image signal is the 24-bit color image signal;and an extraction means for extracting only a Y component from the imagesignal converted to the YIQ format, the image conversion means outputsthe image signal to the first addition means without the conversion ifthe image signal is not the 24-bit color image.
 25. The apparatus asclaimed in claim 24, wherein the image record means comprises: a firstdetermination means for reading out the header information of the imagesignal and determine whether the image is a 24-bit color image; anaddition means for adding the watermark-embedded image signal of the Ycomponent and the IQ components separated by the extraction means if theimage signal is the 24-bit image signal; and a second conversion meansfor converting the added signal of the YIQ format into the RGB format,the image record means storing the processed image signal in a recordmedium.
 26. A digital watermark detection apparatus for detecting awatermark embedded in an image signal, comprising: a pre-processingmeans for strengthening a component of a watermark embedded in theretrieved image signal; an extraction means for converting the digitalwatermark-strengthened image signal from a spatial domain to a frequencydomain and extracting the digital watermark embedded in the imagesignal; a digital watermark generation means for generating respectivedigital watermarks based on a user key and an inherent key for acomparison with the extracted digital watermark, the watermarkgeneration means including: a) a first pseudo-noise code generationmeans for generating a pseudo-noise code based on the user key; b) asecond pseudo-noise code generation means for generating a pseudo-noisecode based on the inherent key; and c) configuration means for arrangingand configuring in a two-dimensional form a watermark comprising thepseudo-noise codes generated from the first pseudo-noise code generationmeans and the second pseudo-noise code generation means, theconfiguration means rotating the watermark from a stream format 360°about a first value of a watermark of a predetermined length to be in aradial format; a correlation calculation means for calculating acorrelation between the generated digital watermark and the extracteddigital watermark from the image signal; and a watermark detection meansfor detecting a watermark embedded in the image signal according to thecalculated correlation.
 27. The apparatus as claimed in claim 26,wherein the image signal is the 24-bit color image, an image signalconversion means for converting a format of the image signal is furtherincluded before carried out by the pre-processor for strengthening acomponent of the digital watermark embedded in the image signal, and theimage signal conversion means includes: a conversion means forconverting the image signal inputted in the RGB format into the YIQformat; and an extraction means for extracting only the Y component ofthe components of the converted YIQ format.
 28. The apparatus as claimedin claim 27, wherein the extraction means includes: a means forconverting from the spatial domain to the frequency domain the imagesignal the digital watermark component is strengthened; a means forextracting in one dimension a region in which the digital watermark isembedded from the converted image signal; and a means for adjusting alength of the extracted one-dimensional digital watermark to apredetermined length.
 29. The apparatus as claimed in claim 28, whereinthe correlation calculations are carried out based on a Formula as blow:Corr=IFFT(FFT(W _(EXT))×conj(FFT(W _(m)))) here, W_(EXT) denotes awatermark extracted from an image signal, and W_(m) respectivewatermarks generated by using a user key and an inherent key, IFFT aninverse fast Fourier transform, FFT a fast Fourier Transform, and conj acomplex conjugate.
 30. The apparatus as claimed in claim 29, wherein thewatermark detection means includes: a means for calculating a peakposition and a peak sharpness from the correlation; and a means fordetermining whether a watermark is included in the image signal based onthe peak position and the sharpness.
 31. The apparatus as claimed inclaim 26, wherein the image signal is the 24-bit color signal, and theR, G, and B channels of the image signal inputted in the RGB format areinputted to the pre-processing means channel by channel.
 32. Theapparatus as claimed in claim 31, wherein the extraction meanscomprises: a means for converting from the spatial domain to thefrequency domain the image signal the digital watermark component isstrengthened; a means for extracting in one dimension a region in whichthe digital watermark is embedded from the converted image signal; and ameans for adjusting a length of the extracted one-dimensional digitalwatermark to a predetermined length.
 33. The apparatus as claimed inclaim 26, wherein the pre-processing means filters a high frequencycomponent of the image signal.
 34. The apparatus as claimed in claim 33,wherein the high frequency filtering is carried out by a high boostfilter, a Laplacian filter, or a Difference of Gaussian (DoG) filter.35. The apparatus as claimed in claim 26, wherein the pre-processingmeans carries out a masking in order to reduce an image component energyand increase a watermark component energy in the image signal.